1. Technical Field
The present invention relates to transducers used in transmitting acoustic energy, and more particularly to megasonic transducers attached to an energy transmitting means using indium.
2. Background Information
It is well-known that sound waves in the frequency range of 0.4 to 2.0 megahertz (MHz) can be transmitted into liquids and used to clean particulate matter from damage sensitive substrates. Since this frequency range is predominantly in the megahertz range, the cleaning process is commonly referred to as megasonic cleaning. Among the items that can be cleaned with this process are semiconductor wafers in various stages of the semiconductor device manufacturing process, disk drive media, flat panel displays and other sensitive substrates.
Megasonic acoustic energy is generally created by exciting a crystal with radio frequency AC voltage radiation. The acoustical energy generated by the crystal is passed through an energy transmitting member and into the cleaning fluid. Frequently, the energy transmitting member is a wall of the vessel that holds the cleaning fluid. The crystal and its related components are referred to as a megasonic transducer. For example, U.S. Pat. No. 5,355,048, discloses a megasonic transducer comprised of a piezoelectric crystal attached to a quartz window by several attachment layers. The megasonic transducer operates at approximately 850 KHz. Similarly, U.S. Pat. No. 4,804,007 discloses energy transmitting members in a megasonic transducer comprised of quartz, sapphire, boron nitride, stainless steel and tantalum.
A problem with megasonic transducers of the prior art is that the acoustic power that can be generated by the megasonic transducer in the cleaning solution is limited to about 10 watts per cm.sup.2 of active piezoelectric surface without supplying additional cooling to the transducer. For this reason, most megasonic power sources have their output limited, require liquid or forced air cooling or are designed for a fixed output to the piezoelectric transducer or transducers. Typically, fixed output systems are limited to powers of 7-8 watts/cm.sup.2. This limits the amount of energy that can be transmitted to the cleaning solution. If more power is applied to the transducer, the crystal can heat up to the point where it becomes less effective at transmitting energy into the cleaning solution. This is caused either by nearing the maximum operating temperature of the crystal or, more often, by reaching the failure temperature of the material used to attach the crystal to the energy transmitting means.
Another problem with prior art cleaning systems that utilize megasonic transducers, is that there is no practical way of replacing a defective transducer once the transducer has been attached to the cleaning system. This means that users have to incur large expenses to replace defective transducers, for example by purchasing a whole new cleaning vessel.